U.S. Pat. Nos. 3,187,300 to Brate and 3,832,762 to Johnson et al. disclose hydrophones having component-to-component seals of epoxy.
U.S. Pat. No. 3,970,878 discloses an acoustic transducer which has no plastic or epoxy parts exposed to the outside of the transducer unit, to avoid electrical leakage caused by the epoxy or plastic absorbing salt from exposure to salt water. However, the disclosed transducer appears, from the patent, and from the technical specifications of the commercial embodiment, to be limited to depths of less than one hundred and fifty feet.
U.S. Pat. No. 4,999,819 discloses an acoustic transducer for use in "deep submergence applications under high hydrostatic pressures." Col. 2, lines 31-32. However, because it is designed for high hydrostatic pressures, it is not as sensitive as desired for shallower water, where streamer cables are usually used. What is needed is a transducer with greater sensitivity, for more accurate readings.
The construction of this transducer also requires, after assembly, curing for ten minutes at 600 degrees centigrade. As a result of such heat, if the piezoelectric cell had been poled before assembly, it then has to be repoled. Piezoelectric cells may be purchased already poled, so it is a waste of time and money to repole them. What is needed is a transducer that does not require a curing which destroys the original poling.
Furthermore, after the repoling, the transducer then has to be stored for at least ten days, to let the piezoelectric cell age, before calibrating the transducer. Because the aging is not linear, most of the aging occurs within the first ten days. The required storage time increases manufacturing time, and increases storage costs. What is needed is a transducer that does not require an aging period after assembly.